To activate the cheat menu, press and hold LEFT SHIFT, then press F11. This will bring up a cheat menu. Enter the following phrases to unlock the cheat listed:

fidel castro: Show dialogues
epitaph: Turn victory condition display on/off
clint: Win the current level
jackal: More ammo
show me all: Show all objects
hollow man: Turn invisibility on/off
timeless: Freeze time
zeus: Press [Alt] to kill enemies with flashlight
medic: Turn hint display on/off
whats my destiny: Turn short briefings on/off
supersonic: Turn sound zone display on/off
powerman: New weapon
schneider: Exit the game